<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
    *{
      margin: 0;
      padding: 0;
    }
    .clear:after{
      content: '';
      display: block;
      clear: both;
    }
    header{
      height: 100px;
      background-color: red;
    }
    .container{
      background-color: yellow;
      margin: 0 120px 0 100px;
      /*overflow: hidden;*/

    }
    .container .center{

      height: 200px;
      float: left;
      width: 100%;
    }

    .container .left{
      height: 200px;
      width: 100px;
      background: blue;
      float: left;
      margin-left: -100%;
      position: relative;
      left: -100px;

    }
    .container .right{
      height: 200px;
      width: 120px;
      background: green;
      float: left;
      margin-left: -120px;
      position: relative;
      right: -120px;
    }
  </style>
</head>
<body>
<header></header>
<div class="container  clear">
  <div class="center">
    内容
  </div>
  <div class="left"></div>
  <div class="right"></div>
</div>
</body>
</html>